The products obtained from pyrolyzing 1,2-dichloroethane, hereafter denoted by DCE, contain in particular small quantities of 1,3-butadiene, of the order of 10 to 100 parts per million (ppm), relative to vinyl chloride. These contents of 1,3-butadiene, denoted hereafter by BD, which are found in the final vinyl chloride are sufficient to inhibit its polymerization and it is necessary to reduce the BD contents to less than 8 ppm in order to make the vinyl chloride polymerizable under industrial conditions.
It has often been found that the content of BD formed during pyrolysis of DCE is lower the purer said DCE used, but it is always necessary to limit the BD content in the vinyl chloride obtained by pyrolysis of DCE in order for vinyl chloride polymerization to be able to proceed normally.
To do this, many methods have been proposed. In the majority of cases, these methods involve, in order to remove the BD, chlorination, hydrochlorination or hydrogenation reactions with or without the presence of catalysts.
Thus, patent FR 1 216 030 proposes to purify the vinyl chloride obtained by pyrolysis of DCE by making the vinyl chloride pass as a mixture with hydrochloric acid, in the vapor phase, over a hydrochlorination catalyst such as HgCl2 or FeCl3. However, this method requires the inclusion of an additional catalytic treatment plant, thereby possibly considerably increasing the production costs, and requires least some of the unconverted DCE to be separated from the pyrolysis products.
In U.S. Pat. No. 3,125,607, anhydrous chlorine is added to the vinyl chloride at a temperature between. −20° C. and 0° C. in a ratio of chlorine to BD to be removed of about 5/1. The BD content passes from 200 ppm to almost 0 ppm after 30 minutes of chlorination.
However, this method has the drawback of including an additional distillation for removing the chlorinated products from the BD, the excess chlorine and various impurities liable to be formed during BD chlorination.
U.S. Pat. No. 3,125,608 discloses a method of purifying gaseous vinyl chloride containing BD, by heating said vinyl chloride with hydrogen to a temperature ranging from 60° C. to 250° C. in the presence of a catalyst consisting of Pd supported by activated alumina.
This method has the drawback of including an additional hydrogenation plant.
According to the method described in U.S. Pat. No. 3,142,709, the vinyl chloride is purified of BD by bringing liquid vinyl chloride into contact with an amount of anhydrous hydrochloric acid representing from 0.5 to 5% by weight of the vinyl chloride for a period ranging from 2 minutes to 5 hours. However, this method applies only to the vinyl chloride already separated from the other pyrolysis products and involves an additional distillation.
Patent FR 1 602 522 discloses a method for obtaining vinyl chloride immediately capable of polymerization, which consists, starting from the raw product derived from the pyrolysis of DCE, in condensing, under a pressure greater than atmospheric pressure, said raw product in order to obtain a liquid phase that is left to mature for at least two hours at a temperature between 0° C. and 100° C. before separating the constituents thereof.
According to that method, the liquid phase after such a treatment contains practically no BD. In practice, this liquid phase remains in tanks.
This method applies most particularly to pyrolysis products containing at most 10 ppm BD relative to vinyl chloride.
When the pyrolysis products contain larger amounts of BD, such as for example at most 20 ppm, the process is carried out in the following manner.
After the prior condensation step, which is carried out by suitably cooling the gaseous pyrolysis products maintained at a pressure above atmospheric pressure, a gas phase is obtained which is in equilibrium with the liquid phase and contains a certain amount of BD. The matured liquid phase is used as liquid for scrubbing said gas phase, so as to absorb the residual BD. It then undergoes further maturing for at least two hours. Thus, by repeating this operation, in one or more columns, depending on the BD content in the raw pyrolysis product, it is possible to completely purify vinyl chloride of BD.
Although giving satisfactory results, these various methods of operation have many drawbacks.
For low BD contents, this method applies only to the liquid phase obtained after condensation of the raw product derived from the pyrolysis of DCE and does not take account of the BD contents of the gas phase that may fluctuate and consequently be found in the final vinyl chloride.
When the amounts of BD are higher, at least one operation of scrubbing the gas phase with the liquid phase matured for at least two hours is carried out, which liquid scrubbing phase is again matured for at least two hours.
This process involves many additional items of equipment and the long residence times result in large volumes of inflammable and toxic products under pressure.